Linear accelerators are devices which accelerate charged particles along a linear path through exposure of the charged particles to time-dependent electromagnetic fields. Since the first testing of linear accelerators by Rolf Wideroe in 1928, linear accelerator technology has experienced significant advancements, perhaps most dramatically following the advancements in microwave technology experienced as a result of World War II radar research. Today linear accelerators represent a powerful tool for nuclear and elementary particle research, and also have been applied to commercial applications.
A linear accelerator delivers energy to a beam of charged particles through application of an electrical field. An early form of linear accelerator, electrostatic linear accelerators, utilize a constant electrical field to deliver energy. Each charged particle accelerated by an electrostatic linear accelerator acquires an energy equal to the product of the potential drop across the linear accelerator and the electric charge of the accelerated particle. The energy of particles is therefore measured in units called "electron volts" (eV). The ability of electrostatic linear accelerators to deliver energy to charged particles is limited by the potential difference that can be maintained by the linear accelerator.
Radio frequency (RF) linear accelerators avoid this limitation by applying a time-varying electric field within a vacuum-maintaining resonance chamber to a charged-particle beam that has been modified to: arrive in "bursts" of charged particles; and only at times in which the polarity of the electrical field is appropriate to accelerate the charged particles in the desired direction. For such a linear accelerator to properly function, the charged-particle beam must be properly phased with respect to the fields, and must maintain synchronization with the fields. Particle accelerators functioning under these principles have been termed "resonance accelerators," and come in a number of configurations, including: linacs, in which the charged particles travel in a straight line; cyclotrons, in which the charged particles travel along a spiral orbit path; and a synchrotron, in which the charged particles travel along a circular orbit path.
Drift tube linacs, or "DTLs," are one form of resonance accelerator. DTLs utilize a series of drift tubes located within a resonance chamber, and through which the charged-particle beam pass, to shield the bursts of the charged-particle beam from exposure to the time-varying electric field during times when the polarity of the field would accelerate the charged particles in a direction opposite that which is intended. Due to the shielding provided by the drift tubes, the bursts of the charged-particle beam are exposed to and accelerated by the field only during passage through the gaps between the drift tubes, and only in the intended direction. Because charged particles are accelerated during passage through each gap, the velocity of the charged particles is greater in each successive drift tube through which the particles pass. The increased velocity of the charged particles in each successive drift tube requires a commensurate increase in the length of successive drift tubes to ensure shielding of the charged particles along the entire distance traveled by the charged particles while the polarity of the accelerating field is the opposite of that desired.
Drift tubes in a DTL generally contain focusing/defocusing magnets, such as quadrupole magnets, which maintain the size and alignment of the charged-particle beam through the DTL. One side-effect of the operation of a DTL is the generation of heat within the resonance chamber and particularly within the drift tubes. This heat can cause the expansion of drift tube components and thereby modify the geometry of the drift tubes and the length of the gaps between successive drift tubes. These modifications may affect the dynamics of the charged-particle beam, including its frequency. While small perturbations in the frequency of the beam may be compensated for, significant perturbations will impair the ability of the RF field to impart energy upon the beam. Excessive heating of the drift tubes can also prove detrimental to the magnets' ability to perform its functions by altering the magnets' parameters, reducing the magnets' strength, or by introducing multipoles that may lead to emmittance growth.
Cooling systems are frequently used in conjunction with DTLs to control drift tube heating and eliminate or reduce the effects of heating on drift tube geometry and magnets. These cooling systems typically circulate a cooling fluid, such as water, through selected components of a DTL. It is known in the prior art that cooling fluid may be circulated through the stems by which drift tubes are attached to the interior wall of a DTL's resonance chamber. U.S. Pat. No. 5,021,741 to Kornely, et al., provides another example of a drift tube cooled by the circulation of a cooling fluid. Drift tube cooling becomes especially difficult in high-energy DTLs, where the accumulation of heat may be far more acute.
The manufacture of drift tubes for a DTL, however, is an expensive and difficult process. Difficulties include the high cost of drift tube materials (e.g. high purity copper), the great precision which must be exercised in construction, and the need to manufacture drift tubes in a wide variety of sizes to accommodate the varying velocities achieved by the charged particles at different points within the DTL. The already expensive and difficult manufacturing process is further exacerbated by requirements to form channels for cooling fluid flow within the drift tubes. A need exists for a drift tube design incorporating channels for cooling fluid flow which can achieve desired drift tube cooling while minimizing the difficulties of drift tube construction.